It is not strange at all but very normal and reasonable. You know, the strong sunlight can burn our skin, while you don't know that it can do the same to our eyes. There are UV rays in sunlights which is very hazardous. What it can do is more than just darkending our skin. It can burn the skin, cause wrinkels and spots, and even skin cancer. As to impact on eyes, if long time exposure to the sunlight, the surface tissue of eyes would be burned. As to more serious problems, it can cause pterygium at the corner of eyes, pinguecula and cataracts. Long term exposure to the UV radiation would increase the risk of developing cataract or accelerate the growth of cataract. All in all, it is much better to wear a good pair of sunglasses which can provide 100% protection from the damages of sunlight. There is no reason for you to refuse the sunglasses.